// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "media/base/bit_reader_core.h"

#include <stdint.h>

#include "base/sys_byteorder.h"

namespace {
const int kRegWidthInBits = sizeof(uint64_t) * 8;
}

namespace media {

BitReaderCore::ByteStreamProvider::ByteStreamProvider()
{
}

BitReaderCore::ByteStreamProvider::~ByteStreamProvider()
{
}

BitReaderCore::BitReaderCore(ByteStreamProvider* byte_stream_provider)
    : byte_stream_provider_(byte_stream_provider)
    , bits_read_(0)
    , nbits_(0)
    , reg_(0)
    , nbits_next_(0)
    , reg_next_(0)
{
}

BitReaderCore::~BitReaderCore()
{
}

bool BitReaderCore::ReadFlag(bool* flag)
{
    if (nbits_ == 0 && !Refill(1))
        return false;

    *flag = (reg_ & (UINT64_C(1) << (kRegWidthInBits - 1))) != 0;
    reg_ <<= 1;
    nbits_--;
    bits_read_++;
    return true;
}

int BitReaderCore::PeekBitsMsbAligned(int num_bits, uint64_t* out)
{
    // Try to have at least |num_bits| in the bit register.
    if (nbits_ < num_bits)
        Refill(num_bits);

    *out = reg_;
    return nbits_;
}

bool BitReaderCore::SkipBitsSmall(int num_bits)
{
    DCHECK_GE(num_bits, 0);
    uint64_t dummy;
    while (num_bits >= kRegWidthInBits) {
        if (!ReadBitsInternal(kRegWidthInBits, &dummy))
            return false;
        num_bits -= kRegWidthInBits;
    }
    return ReadBitsInternal(num_bits, &dummy);
}

bool BitReaderCore::SkipBits(int num_bits)
{
    DCHECK_GE(num_bits, 0);

    const int remaining_bits = nbits_ + nbits_next_;
    if (remaining_bits >= num_bits)
        return SkipBitsSmall(num_bits);

    // Skip first the remaining available bits.
    num_bits -= remaining_bits;
    bits_read_ += remaining_bits;
    nbits_ = 0;
    reg_ = 0;
    nbits_next_ = 0;
    reg_next_ = 0;

    // Next, skip an integer number of bytes.
    const int nbytes = num_bits / 8;
    if (nbytes > 0) {
        const uint8_t* byte_stream_window;
        const int window_size = byte_stream_provider_->GetBytes(nbytes, &byte_stream_window);
        DCHECK_GE(window_size, 0);
        DCHECK_LE(window_size, nbytes);
        if (window_size < nbytes) {
            // Note that some bytes were consumed.
            bits_read_ += 8 * window_size;
            return false;
        }
        num_bits -= 8 * nbytes;
        bits_read_ += 8 * nbytes;
    }

    // Skip the remaining bits.
    return SkipBitsSmall(num_bits);
}

int BitReaderCore::bits_read() const
{
    return bits_read_;
}

bool BitReaderCore::ReadBitsInternal(int num_bits, uint64_t* out)
{
    DCHECK_GE(num_bits, 0);

    if (num_bits == 0) {
        *out = 0;
        return true;
    }

    if (num_bits > nbits_ && !Refill(num_bits)) {
        // Any subsequent ReadBits should fail:
        // empty the current bit register for that purpose.
        nbits_ = 0;
        reg_ = 0;
        return false;
    }

    bits_read_ += num_bits;

    if (num_bits == kRegWidthInBits) {
        // Special case needed since for example for a 64 bit integer "a"
        // "a << 64" is not defined by the C/C++ standard.
        *out = reg_;
        reg_ = 0;
        nbits_ = 0;
        return true;
    }

    *out = reg_ >> (kRegWidthInBits - num_bits);
    reg_ <<= num_bits;
    nbits_ -= num_bits;
    return true;
}

bool BitReaderCore::Refill(int min_nbits)
{
    DCHECK_LE(min_nbits, kRegWidthInBits);

    // Transfer from the next to the current register.
    RefillCurrentRegister();
    if (min_nbits <= nbits_)
        return true;
    DCHECK_EQ(nbits_next_, 0);
    DCHECK_EQ(reg_next_, 0u);

    // Max number of bytes to refill.
    int max_nbytes = sizeof(reg_next_);

    // Refill.
    const uint8_t* byte_stream_window;
    int window_size = byte_stream_provider_->GetBytes(max_nbytes, &byte_stream_window);
    DCHECK_GE(window_size, 0);
    DCHECK_LE(window_size, max_nbytes);
    if (window_size == 0)
        return false;

    reg_next_ = 0;
    memcpy(&reg_next_, byte_stream_window, window_size);
    reg_next_ = base::NetToHost64(reg_next_);
    nbits_next_ = window_size * 8;

    // Transfer from the next to the current register.
    RefillCurrentRegister();

    return (nbits_ >= min_nbits);
}

void BitReaderCore::RefillCurrentRegister()
{
    // No refill possible if the destination register is full
    // or the source register is empty.
    if (nbits_ == kRegWidthInBits || nbits_next_ == 0)
        return;

    reg_ |= (reg_next_ >> nbits_);

    int free_nbits = kRegWidthInBits - nbits_;
    if (free_nbits >= nbits_next_) {
        nbits_ += nbits_next_;
        reg_next_ = 0;
        nbits_next_ = 0;
        return;
    }

    nbits_ += free_nbits;
    reg_next_ <<= free_nbits;
    nbits_next_ -= free_nbits;
}

} // namespace media
